The present invention relates to a nuclear instrumentation monitor for monitoring the power level of a nuclear reactor by measuring an output signal from a neutron detector placed in the nuclear reactor and, more particularly, to a power range monitor system used when the reactor power is in the power operation range.
The instrumentation range of a reactor is divided into a source range, an intermediate range, and a power range. The present invention is applied to a system for monitoring the reactor power in the power range. This system can be suitably applied to a boiling water reactor.
FIG. 1 shows a conventional analog power range monitor system as such a power range monitor system. As shown in FIG. 1, operating voltages are respectively biased from high-voltage power sources 2-1 to 2-n to corresponding neutron detectors 1-1 to 1-n installed in a reactor. The detector signals output from the neutron detectors 1-1 to 1-n are input to a signal output circuits 4-1 to 4-n and an averaging circuit 5 through current.cndot.voltage converters/multiplication circuits 3-1 to 3-n. The output from the averaging circuit 5 is input to an alarm determination/output circuit 6 to be used for alarm determination processing. The output from the averaging circuit 5 is also transmitted as an averaged output from a signal output circuit 7 to an external device.
In the above analog power range monitor system, the current.cndot.voltage converters/multiplication circuits 3-1 to 3-n as negative-feedback circuits constituted by operational amplifiers are used as input circuits for detector signals. The multiplication circuit functions of these current.cndot.voltage converters/multiplication circuits 3-1 to 3-n are used for signal level conversion. The function of calculating an average is realized by the averaging circuit 5 as a negative-feedback circuit constituted by an operational amplifier. The comparison circuit 6 constituting a monitoring function is constituted by an operational amplifier. The signal output circuits 4-1 to 4-n and the signal output circuit 7 are buffer amplifiers constituted by operational amplifiers.
As described above, according to the analog conventional power range monitor system, a dedicated circuit is required to realize one processing function, and signal connection between the respective circuits is performed with electrical signals. For this reason, to perform complicated signal processing, a large circuit is required. In addition, when signals are to be exchanged between the devices, the respective devices must be electrically isolated from each other. For this purpose, an electrical signal isolation device such as an isolation amplifier is placed in each signal route.
In many boiling water reactor plants, the measured value of a core flow rate is obtained from a recirculation flow signal. In a nuclear reactor having two systems of recycle loops, recirculation flow are detected in the form of differential pressure signals, the root squares of the respective detection values are calculated, and the average value of the root squares is obtained. FIG. 2 shows an arrangement for core flow rate calculation in the analog power range monitor system. As shown in FIG. 2, detection signals representing recirculation flow values are sent from recirculation flow differential pressure transmitters (1-A-1 and 1-A-2) to (1-D-1 and 1-D-2) to corresponding average power range monitors 2-A to 2-D. The average power range monitor 2-A receives the detection signals from the recirculation flow differential pressure transmitters (1-A-1 and 1-A-2) through extraction circuits 4-A-1 and 4-A-2 to calculate the root squares of the detection signals, and calculates the average value of the root squares through an averaging circuit 5-A. To use this calculated value as a core flow rate signal for monitoring the reactor power, the two core flow rate signals output from another circuit 2-C are input to a smaller value selection circuit 6-A to select a smaller one of the core flow rate signals, and the selected signal is sent to a rod block monitor 3-A. The average power range monitor 2-B has the same arrangement as that of the average power range monitor 2-A. The average power range monitor 2-B inputs the core flow rate signal calculated by an averaging circuit 5-B and the two core flow rate signals output from another circuit 2-D to a smaller value selection circuit 6-B to select a smaller one of the core flow rate signals, and sends the selected signal to a corresponding rod block monitor 3-B.
As described above, the conventional analog power range monitor system requires extraction circuits 4-A-1 to 4-D-2 and averaging circuits 5-A to 5-D in correspondence with the number of signals to perform extraction and averaging. For this reason, to minimize the number of circuits, signals obtained upon execution of core flow rate signal calculation are connected between the monitors (2-A, 2-C, 2-E) (2-B, 2-D, 2-F).
FIG. 3 shows an arrangement for core flow rate comparison processing in the conventional analog power range monitor system. Core flow rate signals A to D obtained from the detection signals from recirculation flow differential pressure signal transmitters 1-A-1 to 1-D-2 are input to corresponding signal comparison circuits 7-A to 7-D. The signal comparison circuits 7-A to 7-D are connected to each other through signal switching circuits 8-A to 8-D to receive other core flow rate signals upon switching. The outputs from the signal comparison circuits 7-A to 7-D are input to an OR circuit 10 through signal bypass circuits 9-A to 9-D which are opened/closed upon interlocking with the signal switching circuits 8-A to 8-D. The core flow rate signals A to D after calculation are sequentially transmitted and compared in units of segments by switching/controlling the signal switching circuits 8-A to 8-D and the signal bypass circuits 9-A to 9-D.
FIG. 4 shows the arrangement of a digital power range monitor system obtained by digitization of the conventional analog power range monitor system. FIG. 4 shows only a one-system arrangement. The neutron detection signals from neutron detectors (1-A-1 to 1-A-n, 1-C-1 to 1-C-n, 1-E-1 to 1-E-n) are input to average power range monitors (20-A, 20-C, 20-E). The signals from recirculation flow differential pressure signal transmitters (2-A-1, 2-A-2, 2-C-1, 2-C-2) are also input to the average power range monitors 20-A and 20-C. The average power range monitor (20-A, 20-C, 20-E) converts the neutron detection signals into digital signals, obtains the average power of the signals by average calculation, and sends it to a rod block monitor 21 and other monitors. In addition, the average power range monitor converts differential pressure signals into digital signals, obtains a core flow rate signal by root square calculation and average calculation, and sends it to other average power range monitors 20-C and 20-E.
In such a digital power range monitor system, since a microprocessor is used for digital signal processing, complicated calculation can be easily performed, and a large number of signals can be transmitted through one transmission route by using a data transmission means. By using an optical transmission means as this transmission means, electrical isolation of the respective monitors can be easily performed in exchanging signals therebetween.
In replacing the measuring apparatus in an existing plant, a new measuring apparatus must follow the function realized by the existing apparatus in principle. When, however, the analog equipment is to be replaced with digital equipment, since they differ in their characteristics, it is difficult to realize an identical equipment arrangement. More specifically, the analog equipment is poor in complicated calculation but is designed to perform continuous calculation in terms of time because the respective circuits always operate simultaneously. A delay time in calculation is therefore a very short period of time based on a propagation delay time. According to the analog equipment, design principles associated with electrical isolation and function isolation are realized to a minimum as in a scram signal. In the remaining calculation sections, however, to simplify the circuit arrangements, calculation results are not isolated but connected to each other even between redundant devices. For example, in core flow rate signal comparison in FIG. 3, the circuit arrangement is designed to sequentially send and compare signals after calculation in units of segments, as described above.
The digital equipment can easily perform complicated calculations. However, since each calculation is realized by executing a program using the microprocessor, each calculation is executed once for every execution cycle of the program, resulting in discrete processing in terms of time. A calculation delay time is therefore produced. The power range monitor system, in particular, has the function of instantaneously detecting an abnormal rise in reactor power, which is associated with the safety of the reactor, and outputting a scram signal for emergency shutdown of the reactor. The digital equipment must preferentially process this scram signal. In addition, since signal connection between the devices in the digital equipment is performed by data transmission, a large number of signals can be transmitted/received through one transmission means. The equipment arrangement can therefore be simplified. There is, however, a delay time accompanying data transmission.
As described above, when the conventional analog power range monitor system is to be replaced with a digital power range monitor system, some counter-measures are required against a signal processing delay in the digital equipment. Although advanced calculation functions and simplification of equipment arrangement can be realized, since the functions are realized in an intensive form, one device failure may greatly affect the system. For this reason, the influence ranges of such failures must be limited to prevent any problem in terms of system function.
It is an object of the present invention to provide a power range monitor system which can reduce the influences of a delay time due to digital signal processing and a delay time due to data transmission, detect a failure, and prevent a single failure from affecting the overall system, thereby improving the reliability of the system.